The present invention relates to a process for producing tricyclodecanecarboxylic acid esters useful as excellent flavors.
A known method for producing carboxylic acid esters from monoolefins consists of carbonylating the olefins with CO in strong acids by Koch reaction and esterifying the resulting carboxylic acids in acid catalysts. In order to produce esters of tricyclodecanecarboxylic acid (hereinafter abbreviated as TCDC), dicyclopentadiene (hereinafter abbreviated as DCPD) is hydrogenated and the resulting dihydrodicyclopentadiene (hereinafter abbreviated as DHDCPD) is reacted with CO and H.sub.2 O in strong acids such as sulfuric acid to produce TCDC, which is then esterified. A problem with this method is that cycloolefins are so much susceptible to polymerization in the carbonylation reaction that TCDC cannot be obtained in high yield. With a view to avoiding this problem, U.S. Pat. No. 4,602,107 proposed a process for producing TCDC by bringing an inorganic strong acidic catalyst into contact with tricyclo[5.2.1.0.sup.2.6 ]dec-8-yl formate that was obtained by reaction between DCPD and formic acid. The resulting carboxylic acid is esterified by reacting it with a dialkyl sulfate in contact with an aqueous alkaline solution (see U.S. Pat. Nos. 4,374,052 and 4,411,828).
The synthesis of TCDC by the method described in U.S. Pat. No. 4,602,107 proceeds by the following route of reaction: ##STR1## TCDC synthesis in a strong acid by this route of reaction involves rearrangement of the formate ester, which contributes to avoidance of the problem of susceptibility of olefins to polymerization in a strong acid, thereby improving the yield of TCDC that can be obtained. This approach, however, suffers the disadvantage of using a large volume of a strong acid such as sulfuric acid or anhydrous hydrofluoric acid. For example, in order to attain a high yield of TCDC, the strong acid must be used in an amount of at least 5 moles per mole of the reaction product. When sulfuric acid is used as the strong acid, it must be diluted with a large volume of water to liberate and recover TCDC from the reaction solution but this makes it impossible to put the sulfuric acid to another use. On the other hand, anhydrous hydrofluoric acid can be put to another use irrespective of its amount since it can be separated by distillation from the reaction product by making use of its highly volatile nature (b.p. 20.degree. C.).
Noting this fact, the present inventors reviewed the possibility of synthesizing TCDC using anhydrous hydrofluoric acid as a catalyst. As a result, it was found that part of the formate ester used as a starting material decomposed without being converted to TCDC and that the formation of formic acid and high-boiling point materials was unavoidable. The resulting free formic acid formed a mixture with HF, leading to the formation of HF-H.sub.2 O and generation of CO by decomposition of the formic acid. This causes not only the loss of the catalyst HF but also various process troubles such as increased corrosion of the reactor and difficulty in the operation of catalyst regeneration. All these phenomena combine to reduce the process economy of TCDC synthesis significantly.
TCDC must be esterified before it can be used as a flavor. Tertiary carboxylic acids are generally difficult to esterify and the effect of steric hindrance is particularly great in the case of TCDC. In this method described in U.S. Pat. Nos. 4,374,052 and 4,411,828, diethyl sulfate is used as an esterifying agent but it decomposes in the presence of water that forms as a result of reaction. In order to increase the yield of the final product, an aqueous alkaline solution must be added in many divided portions, which is quite a cumbersome operation to perform. Further, the use of expensive diethyl sulfate reduces the economy of the overall process.